Cluster size and composition dependent water deprotonation by free manganese oxide clusters

Abstract

In the quest for cheap and earth abundant but highly effective and energy efficient water splitting catalysts, manganese oxide represents one of the materials of choice. In the framework of a new hierarchical modeling strategy we employ free non-ligated manganese oxide clusters Mn_xO_x+y⁺ (x = 2–5, y = −1, 0, 1, 2) as simplified molecular models to probe the interaction of water with nano-scale manganese oxide materials. Infrared multiple-photon dissociation (IR-MPD) spectroscopy in conjunction with first-principles spin density functional theory calculations is applied to study several series of Mn_xO_x+y(H₂O)_n⁺ complexes and reveal that the reaction of water with Mn_xO_x+y⁺ leads to the deprotonation of the water molecules via hydroxylation of the cluster oxo-bridges. This process is independent of the formal Mn oxidation state and occurs already for the first adsorbed water molecule and it proceeds until all oxo-bridges are hydroxylated. Additional water molecules are bound intact and favorably form H₃O₂ units with the hydroxylated oxo-bridges. Water adsorption and deprotonation is also found to induce structural transformations of the cluster core, including dimensionality crossover. Furthermore, the IR-MPD measurements reveal that clusters with one oxygen atom in excess Mn_xO_x+1⁺ contain a terminal O atom while clusters with two oxygen atoms in excess Mn_xO_x+2⁺ contain an intact O₂ molecule which, however, dissociates upon adsorption of a minimum number of water molecules. These basic concepts could aid the future design of artificial water-splitting molecular catalysts.